Helix coupled impedance transformer and tubes using same



Ju y 1970 H. L. M DOWELL 3,521,115

HELIX COUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Filed Dec. 7.1967' 2 sheets-sneer 1 FIG I +2 6 I 7 9 & 5g INVENTOR HQ 4 v I HUNTER L.M0 DOWELL BYdfvj/maki TORNEY 1 July 21, 1970 H. L. MCDOWELL 3,521,115

HELIX COUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Filed D430. 7,1967 2 Sheets-Sheet 2 FIG. 5

INVENTOR.

HUNTER LfMC DOWELL BY MM QT NEY United States Patent 3 521,115 HELIXCOUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Hunter L. McDowell,Chatham, N.J., assignor to S-F-D Laboratories, Inc., Union, NJ., acorporation of New Jersey Filed Dec. 7, 1967, Ser. No. 688,929 Int. Cl.H01 25/36; H03h 7/38 US. Cl. 3153.5 9 Claims ABSTRACT OF THE DISCLOSUREA helix coupled periodic circuit broadband impedance transformer isdisclosed which includes an array of conductive elements such asparallel conductive bars, with successive Ones of the bars beinginterconnected by a helical structure and disposed over a ground planesuch that the impedance transformer circuit forms a two wire line. Thebars form periodic shunt capacitive elements, whereas the helical turnsbetween successive bars form periodic series inductance in one of thelines of the circuit. The transformers are dimensioned such that theratio of periodic inductance to periodic capacitance changes widely fromone end of the transformer circuit to the other in a continuous manner.On the other hand, the product of period inductance times periodiccapacitance is caused to remain substantially constant throughout thetransformer circuit such that the high frequency cutoff for thetransformer remains relatively constant.

DESCRIPTION OF THE PRIOR ART Heretofore, impedance transformers had beenproposed for matching transmission lines to helix coupled bar circuitsin microwave tubes. In these prior transformers, the circuit consistedof a helical conductor in combination with a horn shaped ground planemember which flared from a relatively low input impedance to arelatively high output impedance of about 200 ohms. While such atransformer may provide a transformation in impedance with a ratio of 4to 1, the transformer becomes impracticable for transforming with higherratios such as 1 to 8, i.e., from 50 ohms to 400 ohms. Another commonway of making a very broad band impedance transformation is by use of anexponentially tapered length of transmission line. For example, a footlength of tapered strip line may be employed to match a 50 ohm coaxialline to a 300 ohm iterative impedance helix coupled bar circuit. Such animpedance transformer has been shown to have a VSWR of about 2 to 1 overa substantial band of frequencies in the low VHF range. However, the 5foot length of the transformer section is excessively long to bepractical for most tube applications. Also, the desired impedancetransformation is from approximately 50 ohms to 400 ohms and, thus, thelength of such a previously proposed tapered strip line would have to besubstantially in excess of 5 feet.

Thus, a need exists for a relatively small impedance transformer capableof providing impedance transformation ratios on the order of 4 or moreand providing a relatively wide band refiectionless match.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved helix coupled impedancetransformer capable of providing relatively high impedance transformingratios over relatively wide bands of frequencies.

One feature of the present invention is the provision of an impedancetransformer comprising an array of conductive elements disposed over aground plane with suc- 3,521,115 Patented July 21, 1970 cessive ones ofthe conductive elements being interconnected by a helix structure andthe helix structure and the conductive elements being dimensioned suchthat the periodic inductance to periodic capacitance ratio changessubstantially from one end of the impedance transformer to the otherend.

Another feature of the present invention is the same as the precedingfeature wherein the product of periodic inductance times periodiccapacitance of the impedance transformer circuit does not differ by morethan 30% over the length of the circuit.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the transformer includes first andsecond circuit stages with a first circuit stage having less turns ofthe helix structure interconnected between successive conductiveelements than in the second stage.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the impedance transformer is providedin combination with a high frequency tube for matching the impedance ofa helix coupled bar type slow wave circuit to the impedance of an inputor output transmission line, such transformer being preferably disposedexternally of the tubes vacuum envelope.

Other features and advantages of the present invention will becomeapparent upon approval of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic, linearized,longitudinal sectional view of a helix coupled slow wave circuitemployed in a microwave tube,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line2--2 in the direction of the arrows,

FIG. 3 is a view of the structure of FIG. 1 taken along line 33 in thedirection of the arrows,

FIG. 4 is a simplified equivalent circuit for the slow wave circuits ofFIGS. 13,

FIG. 5 is a plan view of a portion of the structure of FIG. 2 takenalong line 5-5 in the direction of the arrows,

FIG. 6 is a schematic plan view, partly broken away, of a microwave tubeemploying features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-3, thereis shown a helix coupled bar type slow wave circuit and a microwave tubeusing same. The helix coupled bar type slow wave interaction circuit 1includes an array of conductive bars 2 with successive bars 2 beinginterconnected by a helix structure 3. Conductive leads 4 are tapped offof the helix 3 at suitable periodic intervals as of one or two turns.The leads 4 then connect to the center of the bars 2. The helix 3 issupported by a ceramic bar 5 from the inside wall of the vacuum envelope6 of the tube.

The bars 2 are supported above one conductive ground plane member 7 viathe intermediary of a plurality of ceramic bars 8 which are brazed onone side of the conductive bars 2 as of copper, and brazed on the otherside to the conductive ground plane 7 of the tube envelope structure 6.A suitable insulative ceramic 8 is beryllia which has relatively highthermal conductivity and, thus, provides a good thermally conductingpath from the bars 2 to the tube envelope 6 for removing heat from thebars 2 in use. Also, the bars 2 are disposed over a second conductiveground plane member formed by the cathode electrode structure 9. Theceramic bars 8 which serve to insulate the conductive bars 2 from theconductive ground plane 7 preferably have a width less than the width ofthe conductive bars 2 such that the bars serve to effectively shield rshadow the ceramic insulators 8 from sputtered cathode material thatcould otherwise deposit over these insulating members to short out theradio frequency fields thereon.

The cathode electrode structure 9 may be of the emitting or non-emittingtype. If it is of the non-emitting sole type, the electrons forinteraction with the slow wave circuit are injected into the regionbetween the bars 2 and the sole 9. In case the cathode electrode 9 is ofthe emitting type, it is typically fabricated of a high secondaryemitting ratio material such as beryllia copper, or aluminum oxide. Thehelix coupled bar type slow Wave interaction circuit 1 is typicallyoperated at ground potential to form the anode electrode and a powersupply 11 typically operates the cathode electrode 9 at a potentialsubstantially negative with respect to the anode potential. A magnetstructure, not shown, produces an axially directed magnetic field B,which is directed through the magnetron interaction region 12 defined bythe space between the conductive bars 2 and the cathode electrode 9. p

A helix coupled impedance transformer structure 15, more fully describedbelow, is carried externally of the tube envelope structure 6 fortransforming the relatively low impedance, as of 50 ohms, of a typicalcoaxial transmission line to the relatively high iterative impedance (acharacteristic impedance) as of 400 ohms of the helix coupled bar typeslow wave interaction circuit 1. Impedance transformer 15 includes aprinted circuit board 16 which is mounted to the tube envelope 6 via achannel member 17. The impedance transformer 15 is connected to the slowwave circuit 1 via a conductive lead 18 which passes axially throughbore 19 in the envelope 6 and ceramic window 21 which serves to providea gas tight wave permeable window structure sealing off the bore 19.

Referring now to FIG. 4, there is shown the equivalent circuit for theslow wave structure 1. Slow wave structure 1 forms a two conductortransmission line. One of the conductors is a ground plane member formedby ground plane structures 7 and 9. The other conductor is the helixcoupled bar circuit with the conductive bars 2 providing periodic shuntcapacitive elements 'between the helix and the ground plane members 7and 9. That portion of the helix structure between adjacent tappingwires 4 forms a periodic series inductance of the slow wave circuit. Thecapacitance between the conductive bars 2 provides a periodiccapacitance in series and with the slow wave circuit, such periodiccapacitance being parallel connected with the periodic inductance of thehelix 3. Typically, the capacitance between adjacent conductive bars 2is relatively small compared to the shunt capacitance between the bars 2and the ground planes 7 and 9 and, thus, the slow wave circuit has atypical low pass, forward wave fundamental dispersion characteristic.The circuit is capable of operating down to DC because the helix coupledbars, forming one conductor of the circuit, are isolated by theinsulative members 8 from the ground plane members 7 and 9 and,therefore, the circuit is capable of transmitting DC power.

Referring now to FIG. 5, there is shown the impedance transformerstructure 15. The impedance transformer 15 comprises a two stage helixcoupled bar circuit. More specifically, an array of conductive bars 31,as of copper, formed on an insulative printed circuit board 32 isaffixed over a ground plane member 33 such as a copper plate. A helixstructure 34 interconnects the conductive bars 31 of the array of bars.The transformer 15 is a two stage transformer having a first stage 35and a second stage 36.

In the first stage 35, successive bars 31 are interconnected by only oneturn of the helix 34, whereas in the second stage 36 successive bars 31are interconnected by two turns of the helix 34. The transformer 15 hasan equivalent circuit substantially the same as that of FIG. 4 where thebars 31 provide periodic shunt capacitive elements to the ground plane33, thus, producing periodic shunt capacity in a two conductor circuit.Periodic inductance is provided by those portions of the helix structure34 interconnecting adjacent bars 31 to provide periodic inductance inseries with one of the conductors of the two conductor circuit.

The transformer circuit 15 is arranged such that the iterativecharacteristic impedance of the circuit 15 continuously increases fromthe low impedance end of the circuit to the high impedance end of thecircuit. The iterative characteristic impedance for the transformercircuit 15 is defined approximately by the relation:

where L is the periodic series inductance of the helix circuit 34 and Cis the periodic shunt capacity of bars 31 to the ground plane 33. Thetransformer 15 is arranged such that the ratio of L/ C continuouslyincreases from the low impedance end of the matching transformer circuit15 to the high impedance end thereof. In the transformer 15 of FIG. 5,the low impedance end is at 37 and the high impedance end is at 38. Whenthe transformer 15 is employed for matching the helix coupled barcircuit 1, as depicted in FIGS. l-3, the iterative characteristicimpedance of the transformer 15 at the high impedance end 38 ispreferably equal to the iterative characteristic impedance of the slowwave circuit 1 which is about 400 ohms. Typically, the low impedance end37 of the transformer forms an impedance match to a conventional powertransmission line, such as, coaxial line 39 having a characteristicimpedance of 50 ohms. Thus, at the low impedance end 37 of the circuit15, the series periodic L is relatively low by providing only a singlesmall turn of the helix 34 between adjacent bars 31 and the bars 31 arerelatively long to provide a relatively high shunt capacity. At the highimpedance end of the first stage 35, the turns of the helix 34 arerelatively large in diameter and the bars 31 are relatively short.Likewise, in the second stage of the impedance transformer 36 the lowimpedance end of the second stage gives relatively small inductance 'byproviding two small diameter turns of the helix structure betweenadjacent bars 31 and the bars are relatively long. At the high impedanceend 38 of the second impedance transformer stage 36 the periodicinductance is relatively high by providing two large turns of the helixbetween successive bars 31 and the bars are relatively short to reducethe shunt capacity.

The terminal bars 31 at the ends of the impedance transformer sections35 and 36 are dimensioned to be half length, i.e., half the length theywould have if they were to be progressively longer or shorter than theiradjacent bar as determined :by the tapered length of the bars. Thesehalf length bar sections come about from filter theory. An alternativeconfiguration for the terminal ends of the transformer sections 35 and36 would be to employ terminal bars 31 which are equal to the lengththey would have to be if they were progressively longer and shortedaccording to the tapered lengths, and then employ terminal inductors L(turns of helix 34) which have half the inductance L they would have ifthey were to be progressively larger or smaller. The difference be tweenthese alternative ways of terminating sections 35 and 36 amounts to thatbetween terminating a filter network in the mid-series or mid-shuntarms.

Both the length of the bars 31 and the diameter of the helix 34 aretapered because the cut-01f frequency for the transformer isproportional to the inverse of the square root of the L-C product. Toobtain the desired transformer bandwidth, it is essential that the highfrequency cut-off be kept at approximately the same frequency throughoutthe length of the transformer 15. By approximately the same frequencythroughout, it is meant that the L-C product should not vary by morethan 30% over the length of the transformer circuit 15.

In addition to the periodic series inductance L, the mutual inductancebetween the turns of the helix 34 is having an effect on thetransformation ratio. The existence of mutual inductance is notnecessary to make the circuit work. However, it appears that this mutualinductance is what may be responsible for giving a larger impedancetransformation than calculated originally for an experimentaltransformer section. This comment does not affect the truth, at leastroughly, of the necessity for maintaining the L/ C ratio constant whiletransforming the ratio of L/C.

Several advantages accrue from placing the impedance transformer 15externally of the tube envelope 6. One advantage is that it can bereadily adjusted to prevent undesired reflections from the transformer."In addition, tune mode absorbers may be afiixed to the transformer forabsorbing energy from certain undesired modes of oscillation within thetube. In the transformer 15 of FIG. 5, the circuit has been simplifiedfor the sake of explanation and clarity in the drawings. Typically, eachof the transformer sections 35 and 36 would comprise more bars and moreturns of the helix 34. For example, each stage would typically includeapproximately 17 bars, tapered in the manner as indicated in FIG. 15.Each section would be approximately 3 inches in length such that theoverall length of the complete transformer 15 for transforming 50 ohmsto 400 ohms is approximately 6 inches. This compares quite favorablywith the prior art tapered strip lines which were over 5 feet in lengthfor smaller impedance transforming ratios.

Referring now to FIG. 6, there is shown a microwave tube employing thetransformer 15 of the present invention. The tube includes a cylindricalvacuum envelope 6 surrounding a cylindrical non-emissive cathodeelectrode 9. Slow wave structure 1 curves around the cathode electrode 9in a part circular arc to define a severed nonreentrant slow wavecircuit, and an annular magnetrontype interaction region in the spacebetween the conductive bars 2 and the cathode sole electrode 9. Aconductive circuit sever 41 is disposed between the input end of theslow wave circuit 1 and the output end to provide a non-reentrant slowwave circuit. A pair of two stage impedance transformers 15 are providedfor transforming from 50 ohm input and output coaxial cables 42 and 43,respectively, to the slow wave structure 1. The transformers 15 aredisposed on the end of the tube externally of the vacuum envelope 6.

In operation, signal wave energy to be amplified is applied via inputcoaxial line 42 and impedance transformer 15 to the input end of theslow wave circuit 1. Electromagnetic waves excited on the slow wavecircuit 1 interact with a circulating stream of electrons in themagnetron interaction region between the sole 9 and the bars 2 toproduce an amplified output signal on the slow wave circuit 1. Theoutput signal is extracted from the output end of the slow Wave circuit1 via impedance transformer 15 and fed to a suitable load via outputcoaxial line 43.

The tube of FIG. 6 is especially useful for providing wide bandamplification at relatively low microwave frequencies; such as those inthe VHF frequency range. The microwave tube preferably employs anaxially injected electron beam which moves axially through the magnetroninteraction region to a separate collector electrode, such that noise inthe electron stream is not appreciably coupled to the slow wave circuit,thereby extending the dynamic range of the amplifier into the low signalregime.

The impedance transformer 15 and helix coupled bar type slow wavecircuit 1 are useful in tunable oscillator tubes and amplifier tubes ofeither linear or circular geometry.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention can be madewithout departing from the scope thereof it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a high frequency apparatus, means forming an impedance transformercircuit for connecting a first cir cuit to a second circuit and formatching the impedance of the first and second circuits, the improvementwherein, said impedance transforming circuit includes means forming aground plane conductor, means forming an array of conductive 'barsdisposed overlying said ground plane, means for insulatively supportingsaid bars relative to said ground plane, means forming a conductivehelix structure interconnecting successive ones of said conductiveelements of said array, said conducting elements providing predominantlya periodic shunt capacitance for said impedance transforming circuit,said helix structure providing predominantly a periodic seriesinductance interconnecting successive capacitive elements, said helixstructure and said conductive elements being dimensioned such that theratio of periodic inductance to periodic capacitance changes from oneend of said impedance transforming circuit to the other end thereof toobtain the impedance transformation.

2. The. apparatus of claim 1 wherein the product of the periodicinductance and the periodic capacitance at one end of said impedancetransforming circuit does not differ by more than 30% from that producttaken anywhere along said impedance transforming circuit.

3. The apparatus in claim 1 wherein the ratio of periodic inductance toperiodic capacitance changes in a continuous manner from one end of saidmatching transformer circuit to the other end thereof.

4. The apparatus of claim 1 wherein said impedance transforming circuitincludes a first and second circuit stage, said first circuit stagehaving less turns of said helix structure connected in betweensuccessive bars than in said second stage.

5. The apparatus of claim 1 including, means forming a helix coupled bartype slow wave interaction circuit, said slow wave circuit including anarray of parallel conductive bars, means forming a helix structureinterconnecting successive ones of said bars, and said impedancetransforming circuits being coupled at one end to one end of said slowwave interaction circuit.

6. The apparatus of claim 5 wherein said slow wave interaction circuitincludes an array of thermally conductive ceramic members bonded to saidbars of said slow wave interaction circuit, means forming a metallicsupport structure, and said ceramic members being bonded to said supportstructure to provide a thermally conductive path from said bars of saidinteraction circuit through said ceramic member to said metallic supportfor cooling said slow wave circuit.

7. The apparatus of claim 6 including means forming an evacuatedenvelope structure containing said slow wave interaction circuit, andwherein said impedance matching circuit is disposed outside of saidevacuated envelope structure.

8. The apparatus of claim 6 including means forming a cylindricalcathode electrode structure, said bars of said slow wave interactioncircuit being disposed concentrically with and adjacent to said cathodeelectrode structure to define a curved interaction region therebetweento contain a stream of electrons for electronic interaction between thestream of electrons and radio frequency energy on said slow wave circuitto produce an output signal.

9. The apparatus of claim 7 wherein said bars are 7 8 Wider than saidceramic support members, whereby said 3,020,498 2/1962 Ash et a1. 333-34X bars serve as sputter shields for said ceramic members 3,414,75612/1968 Farney 333--34 X to inhibit coating of said ceramic insulators.

HERMAN KARL SAALBACH, Primary Examiner References Cited 5 S. CHATMON,JR., Assistant Examiner UNITED STATES PATENTS 2,588,832 3/1952 Hansell315 3.5 X 2,828,440 3/1958 Dodds et a1 3153.6 315-3.6; 333-33, 352,987,644 6/1961 Anderson 3153.5

